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Subject and Purpose. The mechanism of destruction of the S-state of a nonlinear high-temperature superconductor as part of a coplanar waveguide has not been properly elucidated as the effect of avalanche-type transition to a highly dissipative state, which was experimentally detected by the authors, takes place. The present work is concerned with the development of an appropriate approach describing kinetics of destruction of the S-state of a nonlinear high-temperature superconductor in a coplanar waveguide with allowances made for an inhomogeneous distribution of the microwave current in the superconducting film strip. Methods and Methodology. Use of I.B. and O.G. Vendiks’ reasoning [2] is made on kinetics of the destruction of the superconducting state of a wide film when a direct current governed by the time-dependent Ginzburg-Landau equation is applied. Keeping unchanged their idea as to the S–N boundary forming in the film strip with the boundary movement to the middle of the strip, the S–N boundary motion equation is obtained for a coplanar waveguide, proceeding in doing this from the motion equation of magnetic flux vortices under certain restrictions specified. Results. The time of S-state destruction has been numerically estimated: 1) for a wide superconducting film of YBa2Cu3O7–d composition, the destruction is by the direct current and 2) for a coplanar waveguide based on the same film, the destruction is by the microwave current. When the superconductivity is small (I / I c ³ 1), the destruction time values in both cases are close to each other within the order of magnitude. Conclusion. It is for the first time that the S-state destruction time in a coplanar waveguide has been expressed in terms of the microwave current distribution in the waveguide. It has been shown that this characteristic linearly depends on the ratio between the critical current and the microwave current amplitude in contrast to a quadratic dependence obtained for a superconducting strip with a direct current.